The present invention relates to the field of microphones, in particular to an optical-interference microphone.
A microphone is a transducer for converting acoustic energy into electrical energy. This electrical signal may have special applications, but generally will be converted back into ordinary sound. If the reproduced acoustic signal is to be sensed as an accurate copy of the original, the microphone should have a bandwidth and dynamic range mimicking that of the human ear. The stringency of these requirements can be appreciated by considering the fact that a soft whisper generates a pressure wave with an amplitude which is only a few parts in 1010 relative to atmospheric pressure, and that pain is incurred only when this amplitude is increased by a factor of 106.
Conventional condensor microphones convert acoustic signals into electrical energy. However, conventional condensor microphones are relatively large in size and are not suitable to be manufactured by micromachining techniques. Such conventional microphones include electrodes and a diaphragm. The mechanical properties of the diaphragm determine the bandwidth of the microphone.
Although conventional condenser microphones to receive acoustic signals have generally been accepted, such a microphone is relatively large, and not readily suitable for being formed or placed on a semiconductor chip. Further, the conventional condensor microphone requires electrodes and a relatively large bias voltage applied thereto.
It would be desirable to have available a microphone that is substantially free of these and other shortcomings of condenser microphones. This application discloses such a microphone.
An object of the present invention is to provide a microphone which can be micromachined and is small in size.
Another object of the present invention is to provide a microphone having a broadband and relatively large sensitivity.
It is still another object of the present invention to provide an optical-interference microphone which includes optical components on a silicon chip.
It is still yet another object of the present invention to provide a microphone system which includes a microphone optically connected to an optical circuit.
An aspect of the present invention provides a microphone which includes a diaphragm having a plurality of holes, a back member opposite the diaphragm, and an air gap formed between the diaphragm and the back member. The diaphragm moves in response to an acoustic signal. In a preferred embodiment, the microphone includes an optical fiber having a fiber core, the optical fiber typically being connected to the back member. In another embodiment, the microphone includes a light source and a photodetector for detecting the varying intensity of reflected light due to motion of the diaphragm. The inventive optical-interference microphone is adapted to be formed on a semiconductor chip.
In yet another aspect of the present invention, a microphone system is provided which includes a microphone, an optical circuit, and an optical fiber connecting the microphone with the optical circuit. The optical circuit includes a laser diode, a coupler, an isolator between connecting the laser diode and the coupler, and a photodetector connected to the coupler.
In yet another aspect of the present invention, a sensitive broadband microphone is provided. The microphone is a miniature device, constructed using standard silicon micromachining techniques, and includes a drum type of structure with a diaphragm (membrane) perforated to control ringing. Motion of the diaphragm is sensed using optical interference methods.
In yet another aspect of the present invention, a design is proposed for a micromachined microphone which utilizes optical interference to sense the sound-induced motion of a thin diaphragm. The light source and photodetector can be included as components of the microphone or can be at a remote location and joined to the microphone with an optical fiber. Bandwidth is primarily established by a gap spacing. Further, sensitivity and bandwidth can exceed that of a conventional condenser microphone.
In yet another aspect of the present invention, unlike conventional microphones, the membrane restoring force is not dominated by the tension in the membrane, but instead is due to the compression of the thin layer of gas between the membrane and backing plate. Pneumatic damping, associated with the instantaneous difference in the inside and outside pressures, is used to control ringing and is set by adjusting the porosity of the membrane.